FIGS. 13A to 13G are plan views of a multi-layer transformer which functions as a conventional noise filter disclosed in Japanese Patent Laid-open Publication No.60-257709. The transformer includes magnetic sheets 1, first coil patterns 2, and second coil patterns 3. The first coil patterns 2 and 3 the second coil patterns 3 provided on each magnetic sheet 1 are arranged parallel to each other and have spiral shapes of 0.25 to 0.75 turn from an upper point of view.
As shown in FIGS. 13B to 13F, the magnetic sheets 1 are stacked, and the first coil patterns 2 are connected to one another to form a first coil 4. The second coil patterns 3 are connected to one another to form a second coil 5. Via-electrodes 6 are provided at both end of each first coil pattern 2 on each magnetic sheet 1, and via-electrodes 7 are provided at both ends of each second coil pattern 3. The via-electrodes 6 and 7 on each magnetic sheet 1 is electrically connected with a through-hole 8 in a magnetic sheet 1 to its corresponding electrodes 6 and 7 on another magnetic sheet 1. Both ends of the first and second coils 4 and 5, i.e., the coil patterns 2 and 3 on the uppermost and lowermost sheets 1 are connected to lead electrodes 9a to 9d. The coil patterns 2 and 3 on the uppermost and lowermost sheets 1 have a spiral shape of 0.5 turn except their ends around to the lead electrodes 9a to 9d. 
As shown in FIGS. 13A and 13G, magnetic sheets 1 are provided on the first coil 4 and the second coil 5.
The first coil 4, the second coil 5, and the magnetic sheets 1 are stacked together to provide a noise filter.
In the conventional noise filter, when a noise in a common mode is applied to the coils 4 and 5, currents flow in the coils in the same direction from an upper point of view. The filter has an impedance increase accordingly, thereby suppressing the noise in the common mode.
However, the conventional noise filter may hardly increase the impedance in the common mode up to a desired level for suppressing noise components. Since the first coil pattern 2 and the second coil pattern on each magnetic sheet 1 have the spiral shapes of 0.25 turn to 0.75 turn, the coil patterns influence each other are short. Accordingly, magnetic flux generated by the first coil 4 and the second coil 5 is too small to emphasize each other, and thus, the filter does not have a large impedance in the normal mode of the filter.
FIG. 14 is an exploded perspective view of another conventional noise filter disclosed in Japanese Patent Laid-Open Publication No.5-101950. The filter includes a coil assembly 101 made of magnetic sheets having large magnetic permeability and lead assemblies 102 and 103 made of magnetic sheets having small magnetic permeability. The lead assemblies 102 and 103 are provided on both, upper and lower, surfaces of the coil assembly 101. A first coil consists mainly of conductors 108a and 109a which are electrically connected to each other with a through-hole 106a. Similarly, a second coil consists mainly of conductors 108b and 109b which are electrically connected to each other with a through-hole 106c. The noise filter has a small impedance for a normal component at the lead assemblies, thus suppressing a common mode noise without seriously disturbing a signal.
The conventional noise filter suppresses the common mode noise by having a small impedance for the normal component throughout the coil. The noise filter further suppresses the common mode noise by having a large impedance for a common component in the coil assembly 101 including the sheets having the large magnetic permeability. In order to have the large impedance for the common component, the filter needs to include tens of coil patterns of less than one turn stacked. This structure increases a number of production steps including fabricating through-holes and printing coil patterns, and they are assembled complicatedly. Such an intricate structure of the noise filter often suffers from open faults and short-circuits, hence having a declining efficiency of its production.